In recent years the laminate core panels of a honeycomb sandwich construction had become increasingly popular in the manufacture of structural panels. The honeycomb sandwich panels comprise a pair of spaced face sheets with a honeycomb core positioned between the face sheets, and with the honeycomb bonded to the face sheet. These honeycomb panels are lightweight and able to withstand considerable compressive loads along the axis of the honeycomb. They are however limited in the amount of bending and sheer loads that can be carried because the bonding between the face sheets and the honeycomb is essentially a line contact with limited area for bonding the honeycomb and the face sheet.
Other core materials that are used in structural sandwich construction include PVC foam and balsa wood. PVC foam is easily deployed between the face sheets and initially forms a lightweight sandwich. However the foam has limited strength and in applications such as a boat hull, repeated pounding and stress encountered in rough weather eventually breaks down the foam structurally and what remains is a loose sand like powder positioned between the face sheets. Balsa wood has been tried as a core material for sandwich construction in boat hulls, but the Balsa wood has an affinity for moisture which eventually greatly adds to the weight of the overall structure making it a marginal material at best.
Examples of core materials for sandwich panels are illustrated in U.S. Pat. Nos. 4,348,442; 3,950,585; 3,168,432; 3,975,882; 3,966,013; 3,819,007; 3,544,417; 3,403,068; and 3,060,561. None of the above listed patents disclose nor anticipate the closely packed closed cell structure of the present invention with the closed cells bonded one to another and to the face sheets to form an incredibly strong sandwich structure capable of accommodating much higher sheer and bending loads.